If this is the case, says Miller, the research by Barnard suggests that the risk of free radical production in most is likely to cancel out the benefit of sun protection, and the Therapeutic Goods Administration (TGA) should not permit their use.

A spokeswoman for the TGA says the authority is examining the paper by Barnard, but was unable to provide further comment at the time of writing.

The TGA website states that to date, the current weight of evidence suggests that titanium dioxide and zinc oxide nanoparticles in sunscreens do not present a risk because they remain in the outer layer of the skin.

But environment and consumer groups are concerned about the lack of in-vivo studies of sunscreens and have been calling on the TGA to require nano-ingredients in sunscreen be tested, labelled and properly regulated.